Largest lake on Saturn’s moon Titan as smooth as a mirror

London: A new study has shown that the largest lake on Saturn’s moon Titan is as smooth as a mirror, varying in height by less than 3 millimeters, and good enough for skipping rocks on it.

According to a report in New Scientist, the find, based on new radar observations, adds to a deluge of evidence that the moon’s lakes are indeed filled with liquid, rather than dried mud.

“Unless you actually poured concrete and spread it really, really smoothly, you’d never see something like that on Earth,” said team member Howard Zebker of Stanford University.

Astronomers have waffled on whether Saturn’s largest moon is dry or wet, but the bulk of the evidence points to liquid lakes.

The radar on the Cassini spacecraft, which arrived at Saturn in 2004, turned up dark splotches at Titan’s poles.

The darkness in radar indicates those regions are very smooth, like the signal expected from the surface of a liquid lake.

Spectral data also showed that the apparent lakes seem to be filled with methane and ethane, which would be liquid on Titan’s frigid surface, and “geomorphologically, they just look like lakes,” Zebker said.

But, previous radar observations viewed the apparent lakes at an angle, and therefore did not see bright radar glints reflected back from their surface, leaving open the possibility that the features were dry lake beds or patches of soot.

Now, researchers report seeing just that signal.

In December last year, Cassini pointed its radar straight down over Titan’s largest lake, Ontario Lacus, which spans 235 kilometres at the moon’s south pole.

The reflected signal was so strong, it maxed out the probe’s receiver.

The radar echoes revealed a surface covering thousands of square metres whose height varies by less than 3 millimetres – 10 times as flat as previous measurements were able to reveal.

“It’s very hard to imagine a solid surface that is smooth on the order of millimeters,” lead author Lauren Wye of Stanford told New Scientist.

This provides strong evidence that the lake is currently liquid, not dried mud.

“If you’ve ever walked outside and seen an area on the ground where there’s mud and the water dries up, even that is pretty flat – but you get cracks in the mud and pieces that curl up,” Zebker said. “You never see anything as smooth as what we’re inferring for Titan’s surface,” he added.

Confirming the presence of liquid on Titan adds to the long list of similarities between Titan and Earth.